1. Field of the Invention
The present invention relates to a wireless communication system in which a portable device carried by a user transmits a wireless signal to an in-vehicle device mounted on a vehicle by a spread spectrum method.
2. Description of the Related Art
In a wireless communication by a spread spectrum method, an original signal is spread by a spreading code at a transmitting end to generate a spread signal, and the spread signal is despread by the spreading code at a receiving end. In the spread spectrum method, the receiving end needs a synchronous acquisition of the spread signal and the spreading code. The synchronous acquisition can be performed, for example, by using a sliding correlator or a matched filter. Methods of performing a synchronous acquisition using a sliding correlator are disclosed, for example, in JP-A-5-219011 and JP-A-8-237170.
U.S. 2009/0323772 A (corresponding to JP-A-2010-011061) by some of the inventors of the present application discloses a wireless communication system for reducing a time required for a synchronous acquisition and improving a responsiveness. In the wireless communication system, an in-vehicle device transmits a reference period to a portable device before a wireless communication by a spread spectrum method and a synchronous adjustment is performed. The potable device transmits a wireless signal that is spread based on the reference period. The in-vehicle device performs a synchronous acquisition using a sliding correlator or a matched filter within a range that is set in view of variation in delay time generated in the synchronous adjustment.
When the wireless communication system is applied to a smart entry system, a time required for the synchronous acquisition can be reduced, and a responsiveness of the wireless communication system can be improved.
In general, a smart entry system coexists with a keyless entry system in which a vehicle door is locked and unlocked by a remote control by a user. Also in the keyless entry system, a spread spectrum modulation is performed using the same spreading code as the smart entry system. Lock and unlock of the vehicle door by the keyless entry system is based on an operation by a user and needs to be performed prior to a process by the smart entry system.
In the wireless communication system, the in-vehicle device performs the synchronous acquisition of a received signal within the range set in view of variation in the delay time. Thus, when the in-vehicle device receives a keyless signal that is transmitted regardless of the reference period instead of a smart signal, the in-vehicle device may fail in the synchronous acquisition. Thus, a process by the keyless entry system may delay and a responsiveness to an operation by a user may be reduced.